In a cellular radio communication system, when a terminal moves from the cell currently providing services (hereinafter, “serving cell”), to another cell or when the network needs to intentionally re-connect the terminal to another cell to balance network load among cells, handover is necessary to change the serving base station to which this terminal is connected, to another base station. The cell to which a terminal is currently connected and which provides services is also referred to as a “source cell.” The cell to which a terminal prepares to connect is referred to as a “target cell.”
Before a handover, a terminal generally measures channel qualities for candidate cells, and selects, for example, the cell of the best channel quality as the target cell. The candidate cells are also referred to as “neighboring cells” of the source cell.
There are three types of handover, including: intra-frequency handover; inter-frequency handover; and inter-RAT handover. Intra-frequency handover refers to handing over a terminal to a target cell that employs the same radio access technology (“RAT”) and frequency band as the source cell. Further, inter-frequency handover refers to handing over a terminal to a target cell that employs the same radio access technology as the source cell and a different frequency band from the source cell. Furthermore, inter-RAT handover refers to handing over a terminal to a target cell that employs a different radio access technology from the source cell. In a cellular radio communication system accommodating cells of various radio access technologies and various carrier bands, the three types of handover enable terminals to move while receiving services.
In association with the three types of handover, measurement performed for handover can also be classified into three types, including: intra-frequency measurement for intra-frequency handover; inter-frequency measurement for inter-frequency handover; and inter-RAT measurement for inter-RAT handover. Intra-frequency measurement refers to measuring those neighboring cells that employ the same RAT and the same carrier frequency as the source cell. Further, inter-frequency measurement refers to measuring the neighboring cells that employ the same RAT as the source cell and a different frequency band from the source cell. Furthermore, inter-RAT measurement refers to measuring the neighboring cells that employ a different RAT from the source cell.
For intra-frequency measurement, a terminal does not need to tune its receiver to a frequency other than the frequency of the serving cell to measure neighboring cells. For inter-frequency and inter-RAT measurements, there are the following two approaches.
As the first approach, if a terminal has a function for supporting reception from two or more frequency bands at the same time, the terminal can use one of receiving circuits to measure another cell that employs a different carrier frequency or different RAT, and use the other receiving circuit to tune its frequency to the frequency of the serving cell to continue data transmission in the serving cell.
As the second approach, if a terminal does not support reception from a plurality of frequency bands at the same time, the terminal needs to tune its receiving circuit from the frequency of the source cell to another frequency of a neighboring cell or to a frequency of another RAT, to perform inter-frequency or to perform inter-RAT measurement, and needs to re-tune the receiving circuit back to the frequency of the source cell after measurement in order to continue data transmission in the source cell. This procedure for a terminal that does not support reception from a plurality of frequency bands at the same time to perform inter-frequency measurement or inter-RAT measurement requires that a terminal sets an idle period to continue data transmission in the serving cell. Furthermore, in such an idle period, it is assumed that the serving base station and the terminal are synchronized so as not to transmit data from the serving base station to the terminal.
As the technology for generating idle periods in the current, so-called third generation cellular radio communication system, which is referred to as a “universal mobile telecommunications system (“UMTS”),” compressed mode is known. In compressed mode, the idle periods (hereinafter, also referred to as “gaps”) for inter-frequency measurement or inter-RAT measurement, are allocated by the serving base station to a terminal such that the terminal can perform inter-frequency measurement and inter-RAT measurement during the gaps. Compressed mode is generally executed in downlink or in downlink and uplink at the same time. Further, in UMTS, a frame is formed with 15 time slots, and part of the time slots are used as gaps for inter-frequency measurement or inter-RAT measurement, while some of the other time slots are used for data transmission. Furthermore, UMTS employs W-CDMA (Wideband-Code Division Multiple Access) as a multiple access technology, and therefore, a technology of, for example, reducing the spreading factor of data to be transmitted in compressed mode, is introduced such that the data transmission rate of a terminal in compressed mode can be maintained the same as in non-compressed mode. At this time, transmission power is increased upon data transmission in time slots without gaps.
Here, a case will be explained where only a single gap pattern sequence of predetermined gap pattern sequences for gap allocation in LTE (Long Term Evolution) is used. Non-Patent Document 1 discloses a single gap pattern sequence supporting both inter-frequency measurement and inter-RAT measurement, and discloses reporting the gap pattern sequence by layer 3 radio resource control (“RRC”) signaling which is transmitted from the base station to a terminal.
Further, Non-Patent Document 2 discloses signaling a large gap by layer 2 media access control (“MAC”) signaling which is transmitted by the base station to a terminal. The techniques disclosed in Non-Patent Document 1 and Non-Patent Document 2 are kind of simplifications of the gap pattern sequences used in compressed mode in UMTS.
FIG. 1 shows examples of gap pattern sequences in compressed mode. FIG. 1A shows gap pattern sequence 1, FIG. 1B shows gap pattern sequence 2 and FIG. 1C shows gap pattern sequence 3. In UMTS, several gap pattern sequences are defined. When a gap pattern sequence is activated, a terminal transitions to a certain gap according to the predetermined time slots specified by the gap pattern gap sequence. In UMTS, it is possible to “reduce a spreading factor” and “increase transmission power” such that the data transmission rate of a terminal in compressed mode can be maintained.
By using only a single gap pattern sequence or several predetermined gap pattern sequences for inter-frequency measurement or (gap assisted) measurement using gaps such as inter-RAT measurement for terminals that support reception from a single frequency band at a given time, it is possible to facilitate processing in compressed mode. However, there is the following problem if such gap pattern allocation in UMTS is directly applied to a cellular radio communication system that performs shared radio resource allocation in LTE.
In LTE, a packet scheduler (hereinafter “channel-aware packet scheduler”) that manages radio quality of channels and schedules transmission and reception of data, performs shared radio resource allocation not only for terminals that receive non-real time services which allow a comparatively long delay time, but also for terminals that receive real time services which allow a comparatively short delay time. The channel-aware packet scheduler selects a terminal when channel quality of the serving cell is high and before the packet delay time required for real time services expires, and allocates radio resources for data transmission.
Assuming that shared radio resource allocation is performed by a channel-aware packet scheduler as, for example, in LTE, if a fixed gap pattern is applied to inter-frequency measurement or inter-RAT measurement, a terminal in compressed mode that receives optimal channel quality from the serving cell is scheduled for data transmission in some subframes. However, there is a high possibility that this terminal cannot transmit data because these subframes have already been allocated as gaps for measurement by a predetermined gap pattern sequence. As a result, the data transmission rate of the terminal in compressed mode decreases, thereby decreasing throughput.
To minimize the influences upon the data transmission rate and throughput of the terminal, it is possible to apply autonomous gap allocation for allocating gaps to the terminal, only in a period in which the channel quality of the serving cell is low or in a period in which this terminal is less likely to be scheduled for data transmission. In autonomous gap allocation, the base station and a terminal need to share gap-related information such that the base station does not schedule data transmission for a terminal when the terminal is in a gap.
In case where a channel-aware packet scheduler performs shared radio resource allocation, when only some predetermined gap pattern sequences for gap assisted measurement are used, there is a problem that the data transmission rate and throughput for both terminals and the base station decrease as described above. Here, in a predetermined gap pattern sequence, the position and the length of a gap are determined in advance. Therefore, there is a possibility that a predetermined gap overlaps the period in which this terminal cannot be scheduled due to a predetermined gap even if this terminal that transmits data is subjected to scheduling. FIG. 2 shows a decrease of throughput in case where predetermined gaps overlap periods of high channel quality. FIG. 2A shows channel qualities (“CQI's”) of three terminals, and, for example, the solid line represents the channel quality of terminal A, the dotted line represents the channel quality of terminal B and the dashed line represents the channel quality of terminal C.
FIG. 2B shows a result of performing scheduling based on channel quality shown in FIG. 2A. Here, the vertical axis represents the amount of data. Further, FIGS. 2C to E represent gap pattern sequences 1 to 3, respectively, and gap pattern sequence 1 is allocated to terminal A, gap pattern sequence 2 is allocated to terminal B and gap pattern sequence 3 is allocated to terminal C.
FIG. 2F shows a scheduling result in case where the gap pattern sequences shown in FIGS. 2C to E are used. FIG. 2F shows outlined portions in which throughput decreases and FIG. 2B shows decreases from the amounts of data in case where there are no gaps.
Further, the terminal may allocate gaps autonomously. As a specific method for autonomous gap allocation, it is possible to use two CQI values of an instantaneous CQI value and an average CQI value, and two thresholds. The instantaneous CQI value represents the channel quality of the measured serving cell, and the average CQI value is the average of instantaneous CQI values over a certain period. The two thresholds of threshold A and threshold B are thresholds for determining a start and end of measurement mode (i.e. compressed mode), and threshold A is set lower than threshold B.
Autonomous gap allocation is performed in the following steps. A terminal measures an instantaneous CQI value and updates an average CQI value continuously. Further, the terminal reports the measured instantaneous CQI value to the base station on a regular basis to facilitate the operation of the above-described channel-aware packet scheduler. If the average CQI value that is updated in the terminal is lower than threshold A, the terminal starts measurement mode. In measurement mode, if the measured instantaneous CQI value is lower than the average CQI value, then the terminal generates gaps for inter-frequency measurement or inter-RAT measurement. If the instantaneous CQI value is higher than the average CQI value, the terminal may transmit the instantaneous CQI value to the base station such that the terminal can continue data transmission. The terminal finishes measurement mode if the average CQI value is higher than threshold B.
As described above, the terminal can report to the base station that gaps have been allocated, by not transmitting CQI's. That is, depending on whether or not CQI's are transmitted, a terminal and base station can share gap allocation information.
Non-Patent Document 1: “Idle Gaps for Handover Measurements in E-UTRAN,” Ericsson, R2-062134, August 2006
Non-Patent Document 2: “Measurement Gap Scheduling,” Qualcomm, R2-062359, August, 2006